The writing of the two Rabbis quoted below state that the Rabbis changed the start of the day from sunrise to sunset. Rabbi Drazin wrote that "we know for certain that the day began in the Temple at daybreak". Do we really know this for certain?
Chancellor Schorsch wrote that the Rabbis "reconfigured the day" and that it is much better the new way.
Is there evidence that the day started at sunrise in the olden days? If so, when was it changed to begin at sunset?
Did Rashbam interpret Genesis 1:5 as meaning days start at sunrise? If so, why did he rule to observe Shabbat etc. beginning at sunset?
Rabbi Israel Drazin wrote:
It is well known that Jews begin their day in the evening at sunset, not at midnight and not at daybreak, but this was not always so. Many scholars are convinced that the biblical Israelite day started at daybreak. It seems possible that the Judeans who were exiled to Babylon accepted the Babylonian practice of beginning the day with the prior evening.
We know for certain that the day began in the Temple at daybreak and it is assumed that the priests in the Temple retained the ancient practice for as long as the Temple existed. When the Bible states “there was evening and there was morning, one day” in Genesis 1:5, its meaning is literal: God completed what was stated earlier during the “daylight period” and this was followed by evening, and when morning came, the day ended – “one day.” The Hebrew is erev and boker. The first means “evening” and the second “morning” or “daybreak.” http://booksnthoughts.com/why-women-must-start-shabbat-before-men/
Rabbi Ismar Schorsch, Chancellor of the Jewish Theological Seminary wrote:
Finally, the festival calendar clearly alludes to a division of time that regards the evening as part of the day just ended. ... Second, the talmudic innovation of reckoning a day from the eve before suggests a larger view of life. While we may never know what prompted the Rabbis to reconfigure the day, the existential benefit is indisputable. http://www.jtsa.edu/prebuilt/ParashahArchives/5758/bereshit.shtml